The sensors which position an aircraft relative to the mass of air, called anemobaroclinometric sensors, deliver fundamental measurements for the aircraft and its safety. Conventionally, these measurements relate to four physical quantities: total pressure, static pressure, temperature (total or static) and incidence. From these primary measurements, elaborate navigation parameters are constructed such as, for example, the conventional speed of the aircraft relative to the air, the pressure at altitude, the incidence or even the Mach number. Without these parameters, the aircraft cannot fly safely. All of these parameters are processed by a central unit, of anemobarometric type, usually redundant, which constitutes the conventional or primary pathway for processing all the information necessary to the navigation of the aircraft.
Given the fundamental nature of the anemobarometric measurements for the safety of an aircraft, it is important to design a backup or secondary pathway, in order to replace, where necessary, the operational pathway in case of failure of the operational pathway and/or to ensure control of the integrity of the measurements that it performs.
The anemobarometric units, which notably comprise Pitot probes coupled to static pressure probes, have the advantage of being able to directly supply a measurement of the modulus of the conventional speed of the aircraft relative to the air. This information is critical to the piloting of the aircraft. In practice, if this speed is too high, the aircraft may be damaged; if too low, it may stall and drop. By associating this measurement with an estimation of the heading (made, for example, by a magnetometer), it is possible to determine the speed vector of the aircraft relative to the air. The real speed of movement then results from the compounding of the speed of the aircraft relative to the air with the average wind speed.
To ensure that the information delivered by an anemobarometric unit is secured, it is known to implement a backup system which must supply a second estimation of the conventional speed of the aircraft relative to the air. The usual backup systems to this end implement methods identical to those used by the operational pathway, that is to say, based on the use of Pitot probes and static pressure probes.
Thus, the measurements performed by the backup pathway are not independent of those delivered by the operational pathway since they are exposed to common failure modes.
To resolve this problem, it is therefore best to implement a backup system which uses measurement means that are different from those of the operational pathway in order to ensure an independent integrity control and reduce the probability of simultaneous failure of both pathways.
One solution to the abovementioned problem consists in using satellite radio navigation systems, also called GNSS (Global Navigation Satellite System) signals, to measure the route and attitude of the aircraft.
The applicant's French patent No. 01 16561 relating to a “method for improving the determination of the attitude of a vehicle using satellite radio navigation signals”, describes the use of satellite radio navigation signals to measure carrier attitude and heading but restricted to the implementation of at least two antennas that are several wavelengths apart.
This solution presents the drawback of requiring at least two GNSS antennas and increasing the overall dimension in the aircraft. Furthermore it brings difficulties in synchronizing the two antennas to which are also added problems of resolving ambiguity concerning the carrier phase deviation measured from the two distant antennas.
One general limitation to the use of the satellite radio navigation signals for air navigation lies in the vulnerability of the GNSS receivers with regard to the availability of the signals or the various disturbances linked to the propagation environment, such as the interferences, multiple paths or scrambling problems. These disturbances are likely to result in significant measurement biases, phase skips and even dropouts of the phase tracking loops thus rendering the service temporarily unavailable.
Although this vulnerability of the GNSS systems has hitherto slowed down their use as primary navigation instrument for an aircraft, they are perfectly compatible with the performance requirements of a backup navigation system having a sufficient availability and accuracy to control the integrity of the measurements supplied by the main system.